Download Where is Fundamental Physics Heading?

Where is Fundamental
Physics Heading?
Nathan Seiberg
IAS
Apr. 30, 2014
Disclaimer
We do not know what will be discovered.
• This is the reason we perform experiments.
• This is the reason scientific research is exciting.
Today we will review the past and the present and
suggest some possibilities for the future.
• An unusual and almost unprecedented situation in physics
• Two “Standard Models” describing the shortest and the
longest explored distances
• They work extremely well in the range of distances they
describe (no contradiction).
• Excellent arguments that there must be new physics
beyond these models
A similar situation
In 1900, the British physicist Lord
Kelvin gave a lecture “NineteenthCentury Clouds over the Dynamical
Theory of Heat and Light.”
• Michelson–Morley experiment
(eventually led to relativity)
• Black body radiation (one of the
roots of quantum theory)
A similar situation
Some people interpreted this to
mean that physics was almost over.
In fact, Kelvin clearly realized the
significance of these “clouds.”
Here we will discuss today’s clouds.
Often research progresses in steps
• Collect data
• Find a pattern – explain a lot of data using fewer
numbers (parameters).
• Understand the underlying reason – the origin of
the pattern.
• Explore the remaining parameters.
• Collect more data
Useful historical examples:
1. Motion of planets
• Collect data
Assyrian star map
from Nineve
Ptolemaeus
Hipparchus
Motion of Planets
• Find a pattern: heliocentric model
– Sun in the center, the orbits are ellipses
– The parameters are the radii
Copernicus
Galileo
Kepler
Motion of planets
Looking for a pattern in the parameters
Kepler had a beautiful
mathematical description of
the sizes of the orbits in terms
of the 5 Platonic solids.
This turned out to be the
wrong question.
Motion of planets
Underlying principles
Newton:
classical mechanics,
gravitational force
Motion of planets
Are the remaining parameters stable?
• Newton was concerned that the solar system was
unstable.
• Historian Michael Hoskin about Newton’s view: “God
demonstrated his continuing concern for
his clockwork universe by entering into what we
might describe as a permanent servicing contract”
for the solar system.
Useful historical examples:
2. Chemistry
• Collect data (many people including the alchemists)
Chemistry
• The pattern – the periodic table (Mendeleev)
Chemistry
Underlying reason – structure of atoms, quantum mechanics
The first Solvay Conference (1911)
Solvay Conference (1927)
Chemistry
• Remaining parameters to explain: mass of the
electron, masses of nuclei, …
Two Standard Models
During the past decades two standard models were found:
• In particle physics: a nearly perfect model describing all
physics at distances larger than roughly 10-19 (one tenth
of a billionth of a billionth) meters .
• In cosmology: a nearly perfect model describing all
physics at large distances, up to roughly 1026 meters –
the entire visible Universe.
The Standard Model of particle physics
• Principles: quantum mechanics, special relativity.
Combined into quantum field theory
• Matter particles: electrons, quarks…
• Forces: electromagnetic force, strong nuclear force,
weak nuclear force
• The parameters: masses of particles, strength of
forces
Matter particles
The protons and neutrons are made of
quarks.
There are several different species of
quarks.
The electron is a member of a larger
family of particles.
New pattern to explain
The “periodic table”
of matter particles
The Higgs particle
• This particle gives mass to matter particles and to the
particles of the weak force.
• In 2012 the Higgs particle was discovered at the LHC.
• This was the last discovered element of the Standard
Model. Its mass is the last measured parameter.
Englert and Higgs
The Standard Model is extremely
successful
• Small number of parameters (like particle masses)
explain many experimental results.
• It is not contradicted by any known experiment!
• Unprecedented success
– some quantities can be computed and measured with
accuracy of 10 significant digits!
Open problems
• The standard model exhibits a pattern. It should be
explained
– Origin of particles, the “periodic table”, etc.
– Origin of forces, their number, their strengths, etc.
– Origin of the parameters like masses of particles
• The Higgs particle mass is unstable (more later)
• Neutrino masses
– Zero in the Standard Model
– Experiment: they are nonzero, but extremely small
– The relevant new physics is at much shorter distances.
Standard Model of cosmology
The best current data from:
• Wilkinson Microwave Anisotropy Probe (WMAP), Planck
space telescope
• Survey of supernovae…
Standard Model of cosmology
The best current data from:
• Survey of supernovae
Riess
Perlmutter
Schmidt
Standard Model of cosmology
• The principles: General relativity, big bang
• Composition of the Universe
– Ordinary matter
– Dark matter
– Dark energy
Standard Model of cosmology
These principles together with a
small number of accurately
measured parameters beautifully
explains all measurements.
No contradiction.
A beautiful and coherent picture
of the Universe.
Standard Model of cosmology
We need to understand:
• What is the dark matter?
• What is the dark energy?
• Origin of parameters (explain the pattern)
Standard Model of cosmology
Cosmic inflation: early period of extremely rapid expansion
• Explains some of the pattern
• Recently, BICEP2 measured
the polarization of the
microwave background,
giving strong evidence for
inflation and measuring
new parameters.
The biggest conceptual question
• Merge the two Standard Models
• Combine quantum mechanics and general
relativity (gravity)
• Quantum gravity is most relevant at the Planck
scale – the basic scale of Nature
– 1018 times the proton mass
– 10-34 meters
• Should address the structure of the Universe
when its size was only 10-34 meters – origin of the
Universe
Best candidate: string theory
Enormous and exciting progress with amazing new
insights during the past decades
Challenges:
• We do not understand the principles.
• We need experimental confirmation.
• It might take a long time to reach any of these goals.
Explain the remaining parameters of
these Standard Models
• The existence of these parameters points to a deeper
truth.
• Look for new models with fewer parameters that
explain/predict the parameters of the current Standard
Models.
• Ultimately, we hope to find a fundamental theory that
explains everything with no input parameters.
Einstein
“…there are no arbitrary
constants ... nature is so
constituted that it is possible
logically to lay down such
strongly determined laws that
within these laws only rationally
determined constants occur (not
constants, therefore, whose
numerical value could be
changed without destroying the
theory).”
Explain the remaining parameters
Hierarchy problems
Qualitative question: explain the overall scales.
• In particle physics:
– Why is the scale of particle physics, so much
longer than the Planck length (a factor of 1016)?
• In cosmology:
– Why is the observable Universe so much larger
than the Planck length (a factor of 1060)?
– Equivalently, why is the amount of dark energy so
small relative to the Planck scale?
Stability of the hierarchy
First, we should understand the origin of these hierarchies
between widely different scales.
Second, we should ensure the stability of the hierarchies.
• Recall Newton’s concern about the stability of the solar
system (and his idea about the need for divine intervention).
• Ensure that small changes in the short distance description
does not make a huge difference in the results.
• Some parameters in the Standard Models seem fine tuned.
Instability?
• This structure is not likely
to be found in nature
– it is unstable.
• If we do find this structure,
we should look for an
explanation; e.g. the rocks
are connected …
Hierarchy problems – possible answers
• Our reasoning is wrong
– Recall Newton and the stability of the solar system
• A new stabilizing mechanism
– Recall the connected rocks
• This is the wrong question
– These parameters are environmental
– Recall Kepler’s Platonic solids
A new stabilizing mechanism
• A stabilizing mechanism is expected
to act at energies close to the
energies of the standard model.
• It should lead to new particles in the
current and future experiments (in
particular the LHC).
• Leading candidate is supersymmetry.
Alternatively, something else we
have not yet thought about.
A new stabilizing mechanism
• So far there is no experimental sign
of any of new particle beyond the
Standard Model.
• Perhaps something will be found in
the next run of the LHC.
• If no discrepancy with the Standard
Model, it is likely that there is no
stabilizing mechanism.
This is the wrong question
• The multiverse – the Universe is much larger than we think.
– Many universes
• Different laws of physics in the other universes
– Different mass for the electron, strength for the electric
forces, etc.
– Different elementary particles
– Different number of dimensions
The multiverse
If true, this is a revolution of the scale of the
Copernican revolution.
Not only isn’t the earth special.
Even our universe is not special.
Obviously, many physicists are
reluctant to accept it.
How should we think about physics in
such a setup?
• What are the right questions in this case?
• Is it meaningful to discuss other unobservable universes?
• Our universe is not special. What principle selects it?
– It does not have to be governed by generic laws of
physics (values of parameters).
– It might not even be the most likely universe.
– Perhaps we should argue that of all possible universes
we live in the one that can support life (anthropic
reasoning).
Summary
• We have two extremely successful Standard Models
– Particle physics – short distances
– Cosmology – the whole Universe
• It is clear that there must be new physics beyond
these models.
• One challenge is to unify them – quantum gravity.
• Perhaps a simpler problem: we should explain the
parameters of these models.
Summary
• Some of these parameters are unstable.
• The LHC is going to give some input into this question
soon.
• One option is that experiments in the near future will find
a mechanism to stabilize some of these parameters.
• Alternatively, some physicists believe that some of these
parameters are environmental.
– Their values are accidents specific to our universe.
– They are different in other universes.
We asked: Where is Fundamental Physics Heading?
We do not know, but it is guaranteed to be exciting.
Thank you for your attention!